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Abstract:

Low bandgap, monolithic, multi-bandgap, optoelectronic devices (10),
including PV converters, photodetectors, and LED's, have lattice-matched
(LM), double-heterostructure (DH), low-bandgap GaInAs(P) subcells (22,
24) including those that are lattice-mismatched (LMM) to InP, grown on an
InP substrate (26) by use of at least one graded lattice constant
transition layer (20) of InAsP positioned somewhere between the InP
substrate (26) and the LMM subcell(s) (22, 24). These devices are
monofacial (10) or bifacial (80) and include monolithic, integrated,
modules (MIMs) (190) with a plurality of voltage-matched subcell circuits
(262, 264, 266, 270, 272) as well as other variations and embodiments.

Claims:

1. A monolithic, multi-bandgap, photovoltaic converter, comprising: a
first subcell comprising GaInAs(P) with a first bandgap and a first
lattice constant; a second subcell comprising GaInAs(P) with a second
bandgap and a second lattice constant, wherein the second bandgap is less
than the first bandgap and the second lattice constant is greater than
the first lattice constant, and further, wherein the second lattice
constant is equal to a lattice constant of a InAsyP1-y alloy
with a bandgap greater than the first bandgap; and a lattice constant
transition material positioned between the first subcell and the second
subcell, said lattice constant transition material comprising
InAsyP1-y alloy with a lattice constant that changes gradually
from the first lattice constant to the second lattice constant.

2. The monolithic, multi-bandgap, photovoltaic converter of claim 1,
wherein the lattice constant transition material is grown epitaxially on
the first subcell with a gradually increasing value for y.

3. The monolithic, multi-bandgap, photovoltaic converter of claim 1,
wherein the second subcell is grown epitaxially on the lattice constant
transition material.

4. The monolithic, multi-bandgap, photovoltaic converter of claim 1,
wherein the first subcell is a lattice-matched, double-heterostructure,
comprising homojunction layers of GaInAs(P) clad by InAsyP1-y
cladding layers wherein the InAsyP1-y cladding has a value for
y in a range of o≦y<1, such the InAsyP1-y cladding
layers of the first subcell have a lattice constant equal to the first
lattice constant.

5. The monolithic, multi-bandgap, photovoltaic converter of claim 4,
wherein the second subcell is a lattice-matched, double-heterostructure
comprising homojunction layers of GaInAs(P) clad by InAsyP1-y
cladding layers, wherein the InAsyP1-y cladding has a value for
y in a range of o≦y<1, such that the InAsyP1-y
cladding layer of the second subcell have a lattice constant equal to the
second lattice constant.

6. The monolithic, multi-bandgap, photovoltaic converter of claim 1,
including a InP substrate, and wherein the first subcell is grown
epitaxially on the InP substrate.

7. The monolithic, multi-bandgap, photovoltaic converter of claim 1,
including a tunnel junction positioned between the first subcell and the
second subcell.

8. The monolithic, multi-bandgap, photovoltaic converter of claim 7,
wherein the tunnel junction is positioned between the first subcell and
the lattice constant transition layer.

11. The monolithic, multi-bandgap, photovoltaic converter of claim 1,
wherein the first bandgap is 0.74 eV.

12. The monolithic, multi-bandgap, photovoltaic converter of claim 6,
wherein the first subcell is grown epitaxially on a front surface of the
InP substrate, the lattice constant transition layer is grown epitaxially
on a back surface of the InP substrate, and the second subcell is grown
epitaxially on the lattice constant transition layer.

13. The monolithic, multi-bandgap, photovoltaic converter of claim 12,
wherein the InP substrate is doped with deep acceptor atoms to make the
substrate more electrically insulating than InP, which is not doped with
deep acceptor atoms.

14. The monolithic, multi-bandgap, photovoltaic converter of claim 1,
including a InP substrate positioned between the first subcell and the
second subcell.

15. The monolithic, multi-bandgap, photovoltaic converter of claim 14,
wherein the InP substrate is positioned between the first subcell and the
lattice constant transition material.

16. The monolithic, multi-bandgap, photovoltaic converter of claim 1,
including an isolation layer positioned between the first subcell and the
second subcell.

17. The monolithic, multi-bandgap, photovoltaic converter of claim 16,
wherein the isolation layer is positioned between the first subcell and
the lattice constant transition material.

18. The monolithic, multi-bandgap, photovoltaic converter of claim 16,
including a InP substrate positioned between the first subcell and the
second subcell.

19. The monolithic, multi-bandgap, photovoltaic converter of claim 18,
wherein the InP substrate is positioned between the second subcell and
the isolation layer.

20. The monolithic, multi-bandgap, photovoltaic converter of claim 18,
wherein the InP substrate is positioned between the first subcell and the
isolation layer.

21. The monolithic, multi-bandgap, photovoltaic converter of claim 14,
including a first isolation layer positioned between the InP substrate
and the first subcell, and including a second isolation layer positioned
between the InP substrate and the second subcell.

22. A monolithic, multi-bandgap, photovoltaic converter, comprising: a
InP substrate with a substrate lattice constant; a first subcell
comprising GaInAs(P) with a first bandgap and a first lattice constant,
wherein the first lattice constant is greater than the substrate lattice
constant; a lattice constant transition material positioned between the
InP substrate and the first subcell, said lattice constant transition
material comprising InAsyP1-y alloy with a lattice constant
that changes from the substrate lattice constant to the first lattice
constant; and a second subcell comprising GaInAs(P) positioned behind the
first subcell, said GaInAs(P) of the second cell having a second bandgap,
which is less than the first bandgap, and a second lattice constant.

23. The monolithic, multi-bandgap, photovoltaic converter of claim 22,
wherein the second lattice second lattice constant is equal to the first
lattice constant.

24. The monolithic, multi-bandgap, photovoltaic converter of claim 23,
including a tunnel junction positioned between the first subcell and the
second subcell.

25. The monolithic, multi-bandgap, photovoltaic converter of claim 23,
including an isolation layer positioned between the first subcell and the
second subcell.

27. The monolithic, multi-bandgap, photovoltaic converter of claim 22,
wherein the second subcell is a lattice-matched, double-heterostructure
comprising homojunction layers of GaInAs(P) clad by InAsyP1-y
cladding has a lattice constant equal to the second lattice constant.

28. The monolithic, multi-bandgap, photovoltaic converter of claim 23,
including: a third subcell behind the second subcell, said third subcell
having a third bandgap, which is less than the second bandgap, and a
third lattice constant, which is greater than the second lattice
constant; and a second lattice constant transition material positioned
between the second subcell and the first subcell, said second lattice
constant transition material comprising InAsyP1-y alloy with a
lattice constant that changes from the second lattice constant to the
third lattice constant.

29. The monolithic, multi-bandgap, photovoltaic converter of claim 22,
wherein the second lattice constant is greater than the first lattice
constant, and including a second lattice constant transition material
positioned between the first subcell and the second subcell, said second
lattice constant transition material comprising InAsyP1-y alloy
with a lattice constant that changes from the first lattice constant to
the second lattice constant.

30. A monolithic, integrated, module (MIM), comprising: a plurality of
monolithic, multi-bandgap, photovoltaic converters, each of which
comprises: (i) a first subcell with a first bandgap and a first lattice
constant; (ii) a second subcell with a second bandgap and a second
lattice constant, wherein the second bandgap is less than the first
bandgap and the second lattice constant is greater than the first lattice
constant; and (iii) a lattice constant transition material positioned
between the first subcell and the second subcell, said lattice constant
transition material having a bandgap at least as large as the first
bandgap and a lattice constant that changes from the first lattice
constant to the second lattice constant; and a common substrate with a
substrate bandgap and a substrate lattice constant, said common substrate
being positioned between the first subcell and the lattice constant
transition material of each of the monolithic, multi-bandgap,
photovoltaic converters, wherein the substrate bandgap is at least as
large as the first bandgap and the substrate lattice constant is equal to
the first lattice constant.

31. The monolithic, integrated, module (MIM) of claim 30, wherein the
first subcells are grown epitaxially on a front side of the substrate,
and wherein the lattice constant transition materials and the second
subcells are grown epitaxially on a back side of the substrate.

33. The monolithic, integrated, module (MIM) of claim 30, including a
tunnel junction positioned between the first subcell and the second
subcell of each of the monolithic, multi-bandgap, photovoltaic
converters.

34. The monolithic, integrated, module (MIM) of claim 33, wherein the
tunnel junction is positioned between the first subcell and the
substrate.

35. The monolithic, integrated, module (MIM) of claim 30, including an
isolation layer positioned between the first subcell and the second
subcell of each of the monolithic, multi-bandgap, photovoltaic
converters.

36. A monolithic, integrated, module (MIM), comprising: a plurality of
monolithic, multi-bandgap, photovoltaic converters, each of which
comprises: (i) a first subcell with a first bandgap and a first lattice
constant; (ii) a second subcell with a second bandgap and a second
lattice constant, wherein the second bandgap is less than the first
bandgap and the second lattice constant is greater than the first lattice
constant; and (iii) a lattice constant transition material positioned
between the first subcell and the second subcell, said lattice constant
transition material having a bandgap at least as large as the first
bandgap and a lattice constant that changes from the first lattice
constant to the second lattice constant; and a common substrate with a
substrate bandgap and a substrate lattice constant, said common substrate
being positioned between the lattice constant transition material and the
second subcell of each of the monolithic, multi-bandgap, photovoltaic
converters, wherein the substrate bandgap is at least as large as the
first bandgap and the substrate lattice constant is equal to the first
lattice constant.

37. The monolithic, integrated, module (MIM) of claim 36, wherein the
lattice constant transition layers and the first subcells are grown
epitaxially on a front side of the substrate, and wherein the second
subcells are grown epitaxially on a back side of the substrate.

39. The monolithic, integrated, module (MIM) of claim 36, including a
tunnel junction positioned between the first subcell and the second
subcell of each of the monolithic, multi-bandgaps, photovoltaic
converters.

40. The monolithic, integrated, module (MIM) of claim 39, wherein the
tunnel junction is positioned between the substrate and the second
subcell.

41. The monolithic, integrated, module (MIM) of claim 36, including an
isolation layer positioned between the first subcell and the second
subcell of each of the monolithic, multi-bandgap, photovoltaic
converters.

42. The monolithic, integrated, module (MIM) of claim 41, wherein a
subcell circuit comprising the first subcells is voltage-matched to a
subcell circuit comprising the second subcells.

43. A photovoltaic converter, comprising: a support platform; a
monolithic, multi-bandgap photovoltaic converter structure, comprising at
least two subcells with different bandgaps that were grown inverted on a
substrate in descending order of their respective bandgaps, wherein at
least one of the subcells was grown epitaxially over and lattice-matched
in relation to the substrate, and at least another one of the subcells
was grown lattice-mismatched in relation to the substrate and was grown
after and over the one of the subcells that was grown lattice-matched to
the substrate; wherein the monolithic, multi-bandgap, photovoltaic
converter structure is mounted on the support platform, such that the
subcell with the lowest bandgap is closest to the support platform; and
wherein the substrate has been removed.

44. The photovoltaic converter of claim 43, including a lattice constant
transition layer positioned between the at least one of the subcells that
was grown lattice-matched in relation to the substrate and the at least
another one of the subcells that was grown lattice-mismatched in relation
to the substrate.

45. The photovoltaic converter of claim 44, wherein the lattice constant
transition layer is transparent to radiation that is transmitted by the
at least one subcell that was grown lattice-matched to the substrate.

46. The photovoltaic converter of claim 43, including two subcells that
were grown lattice-matched in relation to the substrate in descending
order of their respective bandgaps before the at least another one of the
subcells was grown lattice-mismatched in relation to the substrate.

47. The photovoltaic converter of claim 46, including a lattice constant
transition layer positioned between the subcell with the lowest bandgap
of the lattice-matched subcells and the at least another one of the
subcells that was grown lattice-mismatched in relation to the substrate.

48. The photovoltaic converter of claim 47, wherein the lattice constant
transition layer is transparent to radiation that is transmitted by the
subcells that were grown lattice-matched in relation to the substrate.

Description:

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This Application is a Divisional Application of, and claims benefit
and priority to U.S. application Ser. No. 10/515,243, entitled "Low
Bandgap, Monolithic, Multi-Bandgap, Optoelectronic Devices" filed Nov.
19, 2004, which is a 371 Application of PCT/US02/16101, entitled
"Low-Bandgap, Monolithic, Multi-Bandgap, Optoelectronic Devices" filed
May 21, 2002, both of which are hereby incorporated by reference in their
entirety.

[0004] It is well known that the most efficient conversion of radiant
energy to electrical energy with the least thermalization loss in
semiconductor materials is accomplished by matching the photon energy of
the incident radiation to the amount of energy needed to excite electrons
in the semiconductor material to transcend the bandgap from the valence
band to the conduction band. However, since solar radiation and blackbody
radiation usually comprise a wide range of wavelengths, use of only one
semiconductor material with one bandgap to absorb such radiant energy and
convert it to electrical energy will result in large inefficiencies and
energy losses to unwanted heat.

[0005] Ideally, there would be a semiconductor material with a bandgap to
match the photon energy for every wavelength in the radiation. That kind
of device is impractical, if not impossible, but persons skilled in the
art are building monolithic stacks of different semiconductor materials
into devices commonly called tandem converters and/or monolithic,
multi-bandgap or multi-bandgap converters, to get two, three, four, or
more bandgaps to match more closely to different wavelengths of radiation
and, thereby, achieve more efficient conversion of radiant energy to
electrical energy. Essentially, the radiation is directed first into a
high bandgap semiconductor material, which absorbs the shorter
wavelength, higher energy portions of the incident radiation and which is
substantially transparent to longer wavelength, lower energy, portions of
the incident radiation. Therefore, the higher energy portions of the
radiant energy are converted to electric energy by the larger bandgap
semiconductor materials without excessive thermalization and loss of
energy in the form of heat, while the longer wavelength, lower energy
portions of the radiation are transmitted to one or more subsequent
semiconductor materials with smaller bandgaps for further selective
absorption and conversion of remaining radiation to electrical energy.

[0006] Semiconductor compounds and alloys with bandgaps in the various
desired energy ranges are known, but that knowledge alone does not solve
the problem of making an efficient and useful energy conversion device.
Defects in crystalline semiconductor materials, such as impurities,
dislocations, and fractures provide unwanted recombination sites for
photogenerated electron-hole pairs, resulting in decreased energy
conversion efficiency. Therefore, high-performance, photovoltaic
conversion cells comprising semiconductor materials with the desired
bandgaps, often require high quality, epitaxially grown crystals with
few, if any, defects. Growing the various structural layers of
semiconductor materials required for a multi-bandgap, tandem,
photovoltaic (PV) conversion device in a monolithic form is the most
elegant, and possibly the most cost-effective, approach.

[0007] Epitaxial crystal growth of the various compound or alloy
semiconductor layers with desired bandgaps is most successful, when all
of the materials are lattice-matched (LM), so that semiconductor
materials with larger crystal lattice constants are not interfaced with
other materials that have smaller lattice constants or vice versa.
Lattice-mismatching (LMM) in adjacent crystal materials causes lattice
strain, which, when high enough, is usually manifested in dislocations,
fractures, wafer bowing, and other problems that degrade or destroy
electrical characteristics and capabilities of the device. Unfortunately,
the semiconductor materials that have the desired bandgaps for absorption
and conversion of radiant energy in some energy or wavelength bands do
not always lattice match other semiconductor materials with other desired
bandgaps for absorption and conversion of radiant energy in other energy
or wavelength bands. Therefore, fabrication of device quality,
multi-bandgap, monolithic, converter structures is difficult, if not
impossible, for some portions of the radiation frequency or wavelength
spectrum.

[0008] This problem has been particularly difficult to solve in the
infrared (IR) portion of the spectrum, where options for suitable,
commercially available substrates on which to grow thin films with the
necessary bandgaps for absorption and conversion of the infrared
radiation to electrical energy are very limited, and where compatible,
i.e., lattice-matched, semiconductor materials with the different
bandgaps needed to absorb and convert different portions of the infrared
spectrum efficiently are also quite limited.

[0009] For example, the group III-V family of semiconductor alloys include
some of the best materials for fabricating photovoltaic converters with
bandgaps in a range of about 0.35 eV to 1.65 eV to absorb and convert
infrared (IR) radiation with wavelengths in a range of about 3.54 μm
to 0.75 μm. Group III-V alloys comprise combinations of binary
compounds formed from Groups III and V of the Periodic Table. These
binary compounds can be alloyed together into various ternary or
quaternary compositions to obtain any desired bandgap in the range of
0.35 eV to 1.65 eV. These alloys also have direct bandgaps (i.e., no
change in momentum is required for an electron to cross the bandgap
between the valance band and the conduction band), which facilitate
efficient absorption and conversion of radiant energy to electricity.
However, InP, which has a lattice constant of 5.869 Å (sometimes
rounded to 5.87 Å) and a bandgap of 1.35 eV, is one of only a few
feasible, commercially available substrate materials with a lattice
constant even close to those lower bandgap Group III-V alloys i.e.,
InP-based or related ternary and quaternary compounds. The lowest bandgap
Group III-V alloy that can be lattice-matched to the 5.869 Å lattice
constant of an InP substrate is Ga0.47In0.53As, which has a
bandgap of about 0.74 eV, which leaves a significant range of lower
frequency, longer wavelength (>1.67 μm), infrared (IR) radiation
that cannot be absorbed and converted to electricity in monolithic
converters in which the semiconductor absorption materials are
lattice-matched to the substrate.

[0010] While the current unavailability of efficient and cost-effective
solar photovoltaic (SPV) converters, especially multi-bandgap,
monolithic, converter devices, capable of absorbing and converting
infrared (IR) radiation in wavelengths greater than 1.67 μm leaves
substantial amounts of energy in the solar spectrum to remain unconverted
to electricity, in state-of-the-art SPV's, it is an even greater problem
for thermophotovoltaic (TPV) devices Infrared (IR) radiation of
wavelengths greater than 1.67 μm comprises a substantial amount of the
energy radiated from blackbodies, and thermophotovoltaic (TPV) converters
are intended to absorb and convert as much radiant energy from
blackbodies to electric power as possible. Therefore, solutions to these
problems, especially if such solutions could enable fabrication of
monolithic converters with multiple bandgaps in infrared (IR) energy
ranges, they would facilitate capture of more electric energy from solar
and/or blackbody radiation.

[0011] U.S. Pat. No. 5,479,032 issued to S. Forrest et al., teaches that
one or more ternary InxGa1-xAs alloys with x>0.53, i.3.,
with band-gaps less than 0.75 eV, can be grown epitaxially on an InP
substrate by using intervening, graded layers of InAsyP1-y
between the InP substrate and the InxGa1-xP (x>0.53) layers.
However, those Forrest et al., patent teachings, which were directed to
pixel detection of near infrared radiation incident on a focal plane for
telecommunications applications, are not useful in SPV and TPV
applications.

SUMMARY

[0012] Accordingly, a general object of the present disclosure is to
provide a monolithic, multi-bandgap, photovoltaic converter for absorbing
and converting infrared (IR) radiation of multiple wavelengths to
electricity.

[0013] A more specific object of this disclosure is to provide a
photovoltaic converter with at least one bandgap less than 0.74 eV to
absorb infrared radiation in wavelengths longer than 1.67 μm and
convert it to electricity.

[0014] An even more specific object of this disclosure is to provide a
electric device quality, multi-bandgap, monolithic, photovoltaic
converter that has at least one lattice-matched (LM),
double-heterostructure (DH) with a bandgap less than 0.74 eV to absorb
infrared (IR) energy in wavelengths longer than 1.67 μm and convert it
to electricity.

[0015] Another specific object of the disclosure is to provide a device
quality, multi-bandgap, monolithic, photovoltaic device with at least one
lattice-matched (LM), double-heterostructure (DH) with a bandgap less
than 0.74 eV, which is not lattice-matched to an InP substrate, but
including a lattice constant transition layer or layers, which is
transparent to infrared radiation wavelengths longer than about 1.67
μm, positioned somewhere between such lattice-matched (LM),
double-heterostructure (DH) and the InP substrate.

[0016] Still another object of this disclosure is to provide a lattice
constant transition layer or layers, which is transparent to infrared
(IR) radiation wavelengths longer than about 1.67 μm, positioned
between two subcells in a multi-bandgap, monolithic device, where the two
subcells are not lattice-matched to each other and at least one of the
subcells has a bandgap, which is less than the bandgap of the other
subcell and is less than 0.74 eV.

[0017] Another object of the present disclosure is to provide one or more
subcells with bandgaps less than 0.74 eV on an InP substrate.

[0018] Another object of the present disclosure is to provide a bifacial,
monolithic, integrated, module (MIM) comprising multiple subcells, at
least one subcell of which absorbs and converts radiation wavelengths
less than 0.92 μm to electricity.

[0019] Another object of the present disclosure is to provide a bifacial,
monolithic, integrated, module (MIM) comprising multiple subcells, at
least one subcell of which absorbs and converts radiation wavelengths
less than 1.67 μm to electricity.

[0020] Another specific object of this disclosure is to provide a method
of voltage-matching a plurality of subcell circuits that have subcells
with different bandgaps less than or equal to 1.35 eV.

[0021] Additional objects, advantages, and novel features of the
disclosure are set forth in part in the description that follows and will
become apparent to those skilled in the art upon examination of the
following description and figures or may be learned by practicing the
embodiments described herein. Further, the objects and the advantages of
the embodiments described herein may be realized and attained by means of
the instrumentalities and in combinations particularly pointed out in the
appended claims.

[0022] To achieve the foregoing and other objects and in accordance with
the purposes of the a present disclosure, as embodied and broadly
described herein, a method of one embodiment described herein may
comprise growing one or more subcell(s) that has a lattice constant
greater than 5.869 Å, either alone or in combination with other
subcells, on an InP substrate by using a lattice constant transition
material between the InP substrate and the subcell(s) that have the
lattice constants greater than 6.869 Å. The lattice constant
transition material can be InAsyP1-y, where y is graded either
continuously or in discrete stepped increments from one (1) to a value at
which the InAsyP1-y has a lattice constant that matches the
lattice constant of at least one of the subcells with a lattice constant
greater than 5.869 Å. The subcell bandgap is lower than the bandgap
of the InP substrate and lower than the bandgap of the
InAsyP1-y, lattice constant transition material. Additional
subcells with even lower bandgaps can also be added, and, if any of such
additional subcells has an even greater lattice constant that cannot be
matched to the first subcell, then one or more additional lattice
constant transition layers can also be added. All of the subcells can be
grown on only one side of the substrate (monofacial) or one or more
subcells can be grown on the front side of the substrate while one or
more other subcells can be grown on the back side (bifacial), using
whatever lattice constant transition layers are necessary to accommodate
the subcell(s) on each side of the substrate.

[0023] Isolation layers can be used between subcells for independent
electrical connection of the subcells, although, in bifacial embodiments,
the substrate can be insulating or semi-insulating to serve as an
isolation layer. Alternately, tunnel junctions can be used for intra-cell
current flow between subcells. Either the monofacial or bifacial subcell
structures can be made in monolithic, integrated, modules (MIMs), which
are particularly useful for voltage-matching a plurality of such
subcells, although the bifacial embodiments are particularly suitable for
such MIM structures and voltage matching. On the other hand, the
monofacial embodiments are particularly useful in ultra-thin devices in
which the substrate is removed.

[0024] To achieve the foregoing and other objects and in accordance with
the purposes of the various embodiments broadly described herein,
embodiments may also comprise a monolithic, multi-bandgap, photovoltaic
converter that has a first subcell comprising GaInAs(P) with a first
bandgap and a first lattice constant, a second subcell comprising
GaInAs(P) with a second bandgap and a second lattice constant, wherein
the second bandgap is less than the first bandgap and the second lattice
constant is greater than the first lattice constant, and further, wherein
the second lattice constant is equal to a lattice constant of a
InAsyP1-y alloy with a bandgap greater than the first bandgap,
and a lattice constant transition material positioned between the first
subcell and the second subcell, said lattice constant transition material
comprising InAsyP1-y alloy with a lattice constant that changes
gradually from the first lattice constant to the second lattice constant.

[0025] In one embodiment, the first subcell is a lattice-matched,
double-heterostructure, comprising homojunction layers of GaInAs(P) clad
by InAsyP1-y cladding layers wherein the InAsyP1-y
cladding has a value for y in a range of o≦y<1, such the
InAsyP1-y cladding layers of the first subcell have a lattice
constant equal to the first lattice constant. The second subcell may be a
lattice-matched, double-heterostructure comprising homojunction layers of
GaInAs(P) clad by InAsyP1-y cladding layers, wherein the
InAsyP1-y cladding has a value for y in a range of
o≦y<1, such that the InAsyP1-y cladding layer of the
second subcell have a lattice constant equal to the second lattice
constant. Either a tunnel junction or an isolation layer is also
positioned between subcells. The InP substrate can be doped with deep
acceptor atoms to make the substrate more electrically insulating, and,
in bifacial structures, this feature allows the substrate to serve as an
electrical isolation between subcells positioned on opposite sides of the
substrate.

[0026] A plurality of the monolithic, multi-bandgap, photovoltaic
converters can also be grown on a common substrate in a monolithic,
integrated, module (MIM), comprising the plurality of monolithic,
multi-bandgap, photovoltaic converters, each of which comprises: (i) a
first subcell with a first bandgap and a first lattice constant; (ii) a
second subcell with a second bandgap and a second lattice constant,
wherein the second bandgap is less than the first bandgap and the second
lattice constant is greater than the first lattice constant; and (iii) a
lattice constant transition material positioned between the first subcell
and the second subcell, said lattice constant transition material having
a bandgap at least as large as the first bandgap and a lattice constant
that changes from the first lattice constant to the second lattice
constant. Either monofacial structures or bifacial structures can be
grown in MIM configurations, but the bifacial structure is particularly
suited to MIM applications. The subcells in MIM structures can be
isolated for independent electrical connection, or tunnel junctions can
be provided. Isolated, independently connected, subcells are particularly
adapted for voltage-matching in MIM structures. There can be more subcell
stacks on one side of the substrate than the other to facilitate such
voltage-matching, where the subcells on one side of the substrate are
lower bandgap than subcell on the other side of the substrate.

[0027] The substrates can also be removed to provide ultra-thin
photovoltaic devices.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The accompanying drawings, which are incorporated in and form a
part of the specification, illustrate a plurality of embodiments of the
present disclosure, and together with the descriptions serve to explain
the principles of the present disclosure. In the drawings:

[0029]FIG. 1 is a diagrammatic illustration of the general, significant
components of a monofacial, inverted, multi-bandgap, monolithic,
photovoltaic device with two lattice-matched (LM), double-heterostructure
(DH) subcells grown on an InP substrate and series connected, wherein the
second subcell, which has a bandgap that is less than the bandgap of the
first subcell and is less than about 0.74 eV, is lattice-mismatched (LMM)
to the InP substrate, but is grown on a transparent, lattice constant
transition layer positioned between the two subcells to accommodate the
lattice mismatch;

[0030]FIG. 2 is a more detailed cross-sectional view of the device of
FIG. 1 showing some of the auxiliary structures and components useful in
an embodiment of the device;

[0031]FIG. 3 is a bandgap versus lattice parameter chart showing bandgap
and lattice constant parameters of semiconductor materials used as
examples in the embodiments of FIGS. 1 and 2;

[0032]FIG. 4 is a detailed cross-sectional view of a monofacial,
inverted, multi-bandgap, monolithic, photovoltaic device similar to FIGS.
1 and 2, but with the subcells isolated electrically for independent
connection;

[0033]FIG. 5 is a simplified cross-sectional view of a photovoltaic
device illustrating more subcells and graded transparent layers according
to one or more embodiments described herein;

[0034]FIG. 6 is a diagrammatic illustration of a variation of the
monolithic, multi-bandgap, photovoltaic converter similar to FIG. 1, but
with the lattice constant transition layer positioned between the
substrate and the first subcell;

[0035]FIG. 7 is a bandgap versus lattice parameter chart showing bandgap
and lattice constant parameters of the semiconductor materials used as
examples in the embodiment of FIG. 6;

[0036]FIG. 8 is a diagrammatic illustration of a variation of the
monolithic, multi-bandgap, photovoltaic converter similar to FIG. 6, but
with an additional lattice constant transition layer and an additional
subcell added to the structure;

[0037]FIG. 9 is a diagrammatic illustration of a bifacial, buried
substrate embodiment in which subcells are grown epitaxially on opposite
faces of the substrate;

[0038]FIG. 10 is a bandgap versus lattice parameter chart showing bandgap
and lattice constant parameters of the semiconductor materials used as
examples in the embodiment of FIGS. 9 and 11;

[0039]FIG. 11 is an illustration of a more complex, bifacial, monolithic,
multi-bandgap, photovoltaic device;

[0040]FIG. 12 is an illustration of another more complex, bifacial,
monolithic, multi-bandgap, photovoltaic device that is particularly
useful for solar photovoltaic (SPV) converter applications;

[0041]FIG. 13 is a bandgap versus lattice parameter chart showing bandgap
and lattice constant parameters of the semiconductor materials used as
examples in the embodiment of FIG. 12;

[0042]FIG. 14 is a cross-sectional view of a bifacial, monolithic
integrated module (MIM);

[0043]FIG. 15 is a schematic diagram of an equivalent electric circuit
showing the voltage-matched electric subcell circuits of the bifacial MIM
in FIG. 14;

[0044] FIG. 16 is a diagrammatic illustration of a monolithic,
multi-bandgap, photovoltaic device similar to FIG. 2, but with an added
stop-etch layer and with the structure mounted on a panel, heat sink,
printed circuit board, or other object; and

[0045]FIG. 17 is a diagrammatic view similar to FIG. 16, but with the
substrate removed.

DETAILED DESCRIPTION

[0046] A schematic diagram of principle components of a monofacial
embodiment of a low-bandgap, monolithic, multi-bandgap (tandem)
photovoltaic (PV) converter 10 according to one embodiment is shown in
FIG. 1 juxtaposed to a corresponding bandgap energy (Eg) profile.
The diagram in FIG. 1 illustrates a cross-section of the PV converter 10
profile in a manner that is conventional in the industry, i.e., not
necessarily in proportion to actual sizes, because actual layer
thicknesses are too small to illustrate in actual proportions. Additional
structural components used to fabricate an example of the PV converter 10
of FIG. 1 are illustrated in FIG. 2, which will be described in more
detail below.

[0047] In the monofacial embodiment or approach illustrated in FIG. 1, all
of the subcells, for example, subcells 22, 24 in FIG. 1, are grown
epitaxially on only one side or face 25 of a substrate 26. Bifacial
embodiments or approaches (not shown in FIG. 1), in which subcells are
grown epitaxially on opposite sides or faces of a substrate, will be
illustrated in other figures and described below.

[0048] The monofacial PV converter 10 illustrated in FIG. 1 is designed
with low bandgap, Group III-V semiconductor alloy materials, especially
for bandgaps below about 0.74 eV, where ternary GaInAs or AlInAs and
quaternary (GaInAsP or AlGaInAs semiconductor alloys do not match the
crystal lattice constant of InP substrates 26. A quick reference to the
lattice parameter versus bandgap chart in FIG. 3 shows that the crystal
lattice constant of InP is about 5.87 Å, as indicated by broken line
12, while the lowest possible bandgap for a Group III-V alloy with that
same lattice constant of 5.87 Å is about 0.74 eV, which is provided
by the ternary alloy Ga0.47In0.53As, as indicated by the broken
line 14. Lattice-matched (LM) materials refers to materials with lattice
constants that are either equal or similar enough that when the materials
are grown expitaxially, one or the other adjacent each other in a single
crystal, any difference in size of crystalline structures of the
respective materials is resolved substantially by elastic deformation and
not by inelastic relaxation, separation, dislocations, or other
undesirable inelastic effects. A lattice-mismatch (LMM) is generally
considered to occur when a second crystalline material being grown on a
first crystalline material has a lattice constant that is not equal to
the lattice constant of the first material and is not lattice-matched as
described above. (The terms "lattice parameter" and "lattice constant"
mean substantially the same thing and are often used interchangeably in
the art and in this description.) Therefore, as shown by broken lines 16,
18 in FIG. 3, any Group III-V alloy with a bandgap less than about 0.74
eV will be a lattice-mismatched (LMM) with an InP substrate. Since a
significant feature of this disclosure is to provide a monolithic,
multi-bandgap, photovoltaic (PV) converter with at least one bandgap less
than about 0.74 eV, such a lattice-mismatch has to be mitigated in order
to avoid the adverse manifestations of lattice strain and stresses caused
by such lattice-mismatch, such as dislocations, fractures, wafer bowing,
rough surface morphologies, and the like.

[0049] Referring again to the exemplary monofacial, monolithic,
multi-bandgap, photovoltaic (PV) converter 10 illustrated in FIG. 1, such
mitigation of lattice-mismatch between a first Group III-V semiconductor
subcell 22 and a second Group III-V semiconductor subcell 24 with a
different bandgap below about 0.74 eV is provided by a lattice constant
transition layer 20 that: (i) has graded (either distinctly stepped
increments or continuously increasing) lattice constants, which span the
difference between the respective lattice constants of the first and
second subcells 22, 24; and (ii) is transparent to infrared wavelengths
longer than those absorbed by the first subcell 22. While a transparent
lattice constant transition layer 20, which is graded to have lattice
constants that vary continuously from the lattice constant of the first
subcell 22 to the lattice constant of the second subcell 24, is
satisfactory for this purpose, a transparent lattice constant transition
layer 20 comprising, discrete or stepped changes in lattice constants
might be preferable. Dislocations in semiconductor crystals are
undesirable, because they facilitate recombination of charge carriers
(electron hole pairs), which is deleterious to the electrical performance
of a semiconductor device.

[0050] In one embodiment, a lattice constant transition layer 20 is a
ternary InAsyP1-y material in which the proportion of As is
gradually increased, either continuously or in discrete increments as
will be discussed in more detail below. One significant feature is that
the InAsyP1-y lattice constant transition layer 20 is
transparent to infrared (IR) radiation wavelengths longer than those
absorbed by the ternary GaxIn1-xAs or optional quaternary
GaxIn1-xAsyP1-y in the first subcell 22, so there is
virtually no loss of energy, or production of heat, in the lattice
constant transition layer 20.

[0051] The monofacial, monolithic, multi-bandgap, photovoltaic (PV)
converter 10 illustrated in FIG. 1 has an inverted structure with the
active subcell layers 22, 24, transparent lattice constant transition
layer 20, and optional other layers (shown in FIG. 2, which will be
described in more detail below), grown epitaxially on one side 25 of a
substrate 26. This structure is called inverted, because the radiation
energy R enters the converter 10 through the substrate 26, so it has to
be transmitted through the substrate 26 before being absorbed and
converted to electricity by the subcells 22, 24. Therefore, as will be
explained in more detail below, the substrate 26 has to be transparent to
all the incident radiation R so that none of the incident radiation R is
absorbed and thermalized or lost as heat before it reaches the subcells
22, 24, where it can be converted to electricity. Likewise, the incident
radiation R transmitted by the substrate 26 should encounter the subcell
22 with the highest bandgap before it encounters the subcell 24 with the
lowest bandgap, because higher bandgap subcells will absorb only the
higher energy radiation (higher frequency and shorter wavelength) and
convert it to electricity while transmitting unabsorbed, lower energy
radiation (lower frequency and longer wavelength). Therefore, any of the
remaining lower energy incident radiation R that is not absorbed and
converted to electricity by the higher bandgap, first subcell 22 will be
transmitted to the lower bandgap, second subcell 24, where at least some,
if not all, of it can be absorbed and converted to electricity. The
amount of the remaining, lower energy, incident radiation that can be
absorbed and converted to electricity by the second subcell 24 will
depend on the particular bandgap of the subcell 24 and the particular
radiation wavelengths in such remaining, lower energy, incident
radiation. Of course, the lattice constant transition layer 20 has to be
transparent to, and not absorptive of, the remaining, lower energy,
incident radiation that is not absorbed by the first subcell 22 so that
all of such remaining, lower energy, incident radiation can reach the
second subcell 24. Further, as will be explained in more detail below,
additional subcells with different bandgaps can also be included in order
to optimize absorption and conversion of various incident radiation
energy levels or bands to electricity.

[0052] The back-surface reflector (BSR) or other spectral control element
28, which can also function as an electrode contact or lateral current
flow element, is deposited on the second subcell 24, as will be described
in more detail below. A spectral control layer 30 would usually be
deposited on the front side 27 of the substrate 26 either to minimize
reflection of incident radiation R, e.g., an anti-reflective coating
(ARC), as is well-known to persons skilled in the art, especially for SPV
converter applications, or to reflect all incident radiation R with
wavelengths lower than those absorbable by the lowest bandgap subcell 24,
especially for the TPV converter applications used for generating
electricity and not heat. These structures and functions will be
discussed in more detail below. The terms front and back, as used in this
description, relate to the direction in which incident radiation
propagates into and through a device or layers in a device. Therefore,
radiation is incident first on the front of a device or layer and
propagates toward the back of the device or layer.

[0053] In converter 10, substrate 26 comprises InP, because, as explained
above: (i) InP has a lattice constant (5.87 Å), which is one of a few
commercially available bulk, single crystal materials that are close to
the lattice constants of Group III-V alloys that have bandgaps less than
0.74 eV (for absorbing infrared radiation wavelengths longer than about
1.67 μm); (ii) InP has a bandgap of about 1.35 eV (see FIGS. 1 and 3),
thus does not absorb, and is transparent to, infrared radiation
wavelengths longer than 0.93 μm; (iii) InP can be doped to be highly
resistive and thereby function as an insulator or semi-insulator, as
described in more detail below; (iv) Lattice-mismatch between InP and
InAsyP1-y or GaInAs(P) materials, which have lower bandgaps
than InP and are used extensively in this disclosure as explained below,
is in compression rather than tension, so lattice-mismatched
InAsyP1-y or GaInAs(P) grown on InP are not so likely to
develop fissures or crack; and (v) Bulk InP crystals are less expensive
than InAs and GaSb. Therefore, the InP substrate 26 is suitable in a
monofacial, inverted PV converter 10 structure for any application in
which the incident radiation R to be converted to electric energy has
0.93 μm and longer wavelengths, such as thermophotovoltaic (TPV) cells
and some solar photovoltaic (SPV) cells as well as infrared detector
devices and multi-bandgap infrared (IR) LED's. However, InP is
susceptible to free carrier absorption of energy, which results in energy
being lost in the form of heat. To minimize or prevent such free carrier
absorption of energy, the InP substrate can be doped with deep acceptor
atoms, such as iron (Fe) or chromium (Cr), to pin the Fermi level deeply
within the bandgap, which makes the InP act more like an insulator or
semi-insulator.

[0054] Subject to accommodations for a contact, buffer, cladding, optical
control elements, and/or other auxiliary layers (not shown in FIG. 1),
which will be described in more detail below, the first subcell 22 is
deposited on substrate 26 with a bandgap Eg1 designed to absorb the
first desired wavelength or frequency band of the incident radiation R
and convert such absorbed radiation to electricity. In one embodiment,
this first subcell 22 is a lattice-matched (LM), double-heterostructure
(DH), InP/GaxIn1-xAs or InP/GaxIn1-xAsyP1-y
with a desired bandgap Eg1, somewhere in a range that is less than
the 1.35 eV bandgap of the InP substrate 26. This first subcell 22 in
FIG. 1 may be grown epitaxially and is lattice-matched to the InP
substrate 26. Please note that "lattice-matched" when used in the context
of a "lattice-matched, double-heterostructure" for a subcell generally
means that the semiconductor materials within the subcell itself are
lattice-matched to each other. Therefore, a subcell can be a
lattice-matched, double-heterostructure, while such subcell may or may
not be lattice-matched to a substrate or to another layer or material in
the device that is not part of the subcell.

[0055] In one embodiment, subcell 22 lattice-matched to the InP substrate
26 comprises InP/Ga0.47In0.53As with a bandgap of about 0.74
eV, because, as shown by the lines 12, 14 in FIG. 3,
Ga0.47In0.53As is the lowest bandgap Group III-V alloy that has
the same lattice constant as the InP substrate 26. Therefore, a
InAsyP1-y lattice constant transition layer 20 can make a
transition from the lattice constant of InP (about 5.87 Å) to a
lattice constant matching a GaxIn1-xAs alloy with a bandgap as
low as about 0.52 eV, i.e., to a lattice constant as high as about 5.968
Å (see lines 15, 17 in FIG. 3), and still be transparent to all
infrared wavelengths that are longer than those absorbed by the 0.74 eV
bandgap of the first subcell 22 (see line 14 in FIG. 3). Of course, the
desired bandgap for the second subcell 24 could also be anywhere between
0.74 eV and 0.52 eV, in which case the InAsyP1-y lattice
constant transition layer 20 can be formulated to provide a back surface
with whatever lattice constant is needed on which to grow the desired
GaxIn1-xAsyP1-y that has such a desired bandgap.

[0056] An example second subcell 24 for use in conjunction with a first
subcell 22 described above, therefore, can be a quaternary
GaxIn1-xAsyP1-y or a ternary GaxIn1-xAs
with a bandgap as low as 0.52 eV. In one embodiment, an example second
cell 24 comprises a lattice-matched, double-heterostructure
InAsyP1-y/GaxIn1-xAs with a bandgap 19 of 0.55 eV and
a lattice constant 21 of about 5.952 Å.

[0057] The lattice constant transition layer 20, as mentioned above,
gradually makes a transition from the lattice constant of the first
subcell 22 to the lattice constant of the second subcell 24, while
remaining substantially transparent to all infrared radiation wavelengths
that are not absorbed by the first subcell 22, as illustrated by the
example PV converter 10 of FIG. 1. In that specific example, the first
subcell 22 has a bandgap (Eg1) 14 of about 0.74 eV and lattice
constant of about 5.87 Å, while the second subcell 24 has a bandgap
(Eg2) 19 of about 0.55 eV and lattice constant of about 5.952 Å,
as explained above. Therefore, the lattice constant transition layer 20
has to make a transition of lattice constants gradually from about 5.87
Å to about 5.952 Å. As shown in FIGS. 1 and 3, adding As to InP
to produce InAsyP1-y increases the lattice constant of the
InAsyP1-y from about 5.87 Å to about 5.952 Å without
decreasing the bandgap of the InAsyP1-y to a level below the
0.74 eV GaxIn1-xAs (x=0.47) of the first subcell 22. Therefore,
the InAsyP1-y lattice constant transition layer 20 remains
transparent to all of the remaining infrared radiation R that is not
absorbed in the first subcell 22 so that it allows all of such remaining
infrared radiation to reach the second subcell 24.

[0058] As also mentioned above, such graded transition of the
InAsyP1-y lattice constant transition layer 20 from the lattice
constant of the first subcell 22 (e.g., 5.87 Å) to the lattice
constant of the second subcell 24 (e.g., 5.952 Å) can be done by
increasing the proportion of As on a gradual continuous basis or, in
incremental discrete steps as illustrated by line 23 in the bandgap chart
in FIG. 1. The stepped lattice constant transition 23 illustrated in FIG.
1 seems to provide better experimental results than gradual, continuous
grading.

[0059] A more specific example of the monofacial PV converter 10 of FIG. 1
with auxiliary layers useful in actual implementation of such a device
for high quality performance characteristics is shown diagrammatically in
FIG. 2. Again, the thicknesses of the various layers are not illustrated
in actual size or thickness proportions in relation to each other.

[0060] The substrate 26 may be InP doped with a deep acceptor element,
such as Fe, (sometimes denoted as InP:Fe or as (Fe) InP) to trap
electrons and thereby suppress or prevent free carrier absorption. The
substrate 26 can be semi-insulating for isolation or p-type for
conducting, as desired for a particular application, and other layers and
components are designated as either n-type or p-type, accordingly to
provide the n/p junctions 34, 48 needed to convert the incident radiation
R to electricity in the subcells 22, 24, respectively. However, p/n
junctions would also work, as is understood by persons skilled in the
art, so these n-type and p-type designations could be reversed by
substituting donor dopants for acceptor dopants and vice versa, which
would be considered equivalent for purposes of this disclosure.

[0061] While the subcells 22, 24 can be simple shallow homojunctions, this
embodiment is particularly conducive to the more efficient,
lattice-matched, double-heterostructure subcells 22, 24 illustrated in
FIG. 2. Specifically, the example first subcell 22 illustrated in FIG. 2
has a n/p homojunction 34 formed by a p-type GaxIn1-xAs base
layer 38 grown epitaxially on an n-type GaxIn1-xAs emitter
layer 36, all of which is sandwiched between front and back cladding
layers 40, 42 of n-type InP and p-type InP, respectively. The InP in the
cladding layers 40, 42 is a different compound than the
GaxIn1-xAs in the homojunction layers 36, 38, but it has the
same lattice constant as the GaxIn1-xAs. Therefore, the first
subcell 22 is a lattice-matched, double-heterostructure. The cladding
layers 40, 42 passivate dangling bonds at terminated GaxIn1-xAs
crystal structures at the front of layer 36 and at the rear of layer 38,
which otherwise function, at least to some extent, as unwanted
recombination sites for minority carriers in the GaxIn1-xAs.
Also, the band offsets between the InP (bandgap of 1.35 eV) and the
GaxIn1-xAs (bandgap of 0.74 eV in this example) repel minority
carriers away from the InP/GaxIn1-xAs interface, which further
reduces such unwanted recombination of minority carriers. Therefore, a
clad subcell, such as the lattice-matched (LM), double-heterostructure
(DH), InP/Ga0.47In0.53As or optional
InP/GaxIn1-xAsyP1-y subcell 22 described above, is
more efficient in converting radiant energy to electricity than non-clad
subcells. This passivation and confinement of minority carriers by the
cladding layers 40, 42 is possible in the monolithic, multi-bandgap PV
converter structures, because the cladding material, InP, has the same
lattice constant (5.869 Å) as, and a higher bandgap than, the
homojunction cell material, GaxIn1-xAs or
GaxIn1-xAsyP1-y (GaxIn1-xAsyP1-y
is lattice-matched to InP when y≈2.2×).

[0062] The second subcell 24 may also be a lattice-matched,
double-heterostructure comprising a homojunction 48 formed by n-type and
p-type layers 50, 52 of either ternary GaxIn1-xAs or quaternary
GaxIn1-xAsyP1-y, but its lattice constant is larger
than the lattice constant of the first subcell 22 and of the InP
substrate 26, as explained above. Consequently, the second subcell 24 is
lattice-mismatched (LMM) in relation to the InP substrate 26 and first
cell 22, and it cannot be clad with InP. However, as explained above in
relation to the lattice constant transition layer 20, InAsyP1-y
can be formulated to have the same lattice constant as the
GaxIn1-xAs or GaxIn1-xAsyP1-y homojunction
layers 50, 52. Therefore, the passivation and confinement cladding layers
54, 56 of the second subcell 24 comprise InAsyP1-y that is
lattice-matched to the GaxIn1-xAs or
GaxIn1-xAsyP1-y homojunction layers 50, 52 to form
the lattice-matched, double-heterostructure of that subcell 24.

[0063] Prior to growing the first subcell 22, a buffer layer 32 of n-InP
about 300 Å thick is deposited first on a surface 25 of the InP
substrate 26 to begin an epitaxial InP growth layer, if needed. If the
InP substrate 26 is doped with a deep acceptor to be electrically
insulating or semi-insulating as explained above, then provisions have to
be made for a front electrical contact 29 and a conductive layer 33 for
accommodating lateral flow of current produced by the subcells 22, 24 to
or from the contact 29. Such a conductive layer 33 could be, for example,
heavily n-doped InP or any other heavily doped material that is
lattice-matched to the InP substrate 26 as well as transparent to all
radiation wavelengths that are transmitted by the InP substrate 26. Then,
the first subcell 22 comprising the lattice-matched,
double-heterostructure of
n-Ga0.47In0.53As/p-Ga0.47In0.53As homojunction layers
36, 38 between the two cladding layers 40, 42 of n-InP and p-InP,
respectively. As is well-known in the art, semiconductor materials are
usually doped with small amounts of elements from an adjacent group of
the Periodic Table of the Elements to provide the majority carriers.
Therefore, an appropriate donor dopant for the Group semiconductor alloy
used can be, for example, sulphur (S) from Group VI, and appropriate
acceptor dopant can be, for example, zinc (Zn) from Group II. The InP
buffer layer 32 grown epitaxially on the InP substrate 26 in this example
is heavily (10-18-10-20 cm-3) n-type doped with sulfur
(S). Then, the InP front cladding layer 40 is grown epitaxially on the
buffer layer 32 to a thickness of about 0.01-0.1 μm, but it is more
lightly doped n-type with, for example, S to a dopant level of about
1016-1020 cm-3. The Ga0.47In0.53As homojunction
layers 36, 38, which lattice-match the InP substrate 26, buffer layer 32,
and cladding layer 40, are grown epitaxially. Therefore, the bandgap of
the first subcell 22 is about 0.74 eV, which absorbs portions of the
incident radiation R with wavelengths of about 1.67 μm and less, as
explained above, although other values of x and other formulations would
also work in alternate embodiments. Lattice-matching quaternary
GaxIn1-xAsyP1-y is also possible. The emitter layer
36 of subcell 22 is grown epitaxially to a thickness in a range of about
0.1-10 μm, and is doped n-type with, for example, S to a dopant level
in a range of about 1016-1020 cm-3. The base layer 38 is
then grown epitaxially to a thickness of about 0.01-10 μm, and doped
p-type to create the n/p junction 34. The p-type dopant, such as Zn in
this example, is at a dopant level of about 1016-1020
cm-3. To complete the lattice-matched, double-heterostructure, first
subcell 22, the back cladding layer 42 is grown epitaxially on the base
layer 38 to a thickness of about 0.01-0.1 μm, and is p-type doped, for
example, with Zn, to a dopant level of about 1016-1020
cm-3.

[0064] Each of the buffer layer 32, conductive layer 33, and/or cladding
layer 40 can all serve any one or more of these functions, individually
or together. Therefore, instead of the three distinct layers 32,33,40
shown in FIG. 2, one or two layers could serve those same functions, if
desired.

[0065] The subcells 22, 24 can be electrically connected together in
series, or they can be electrically isolated from each other, as will be
described in more detail below. For a monolithic, multi-bandgap, PV
device 10 in which the subcells 22, 24 are series connected, a tunnel
junction comprising a layer 44 of heavily p-doped
Ga0.47In0.53As or GaxIn1-xAsyP1-y followed
by a heavily n-doped Ga0.47In0.53As or
GaxIn1-xAsyP1-y layer 46 is deposited and grown
epitaxially on the back cladding layer 42 of the first subcell 22 to
facilitate low-resistive current flow in an ohmic manner between the
first subcell 22 and the second subcell 24. Again, if homojunction layers
36, 38 of subcell 22 comprise Ga0.47In0.53As, as discussed
above, then it may be that x=0.47 in the GaxIn1-xAs of the
tunnel junction layers 44, 46 in order to lattice-match them with the
underlaying InP and Ga0.47In0.53As layers described above,
although other values of x and other formulations would also work. Tunnel
junctions are well-known in the art, but, for purposes of this
embodiment, each tunnel junction layer 44, 46 can be about 0.01-0.1 μm
thick and doped to a level of about 10-18-10-20 cm-3.
Alternative monolithic, multi-bandgap, PV converters with the subcells
22, 24 isolated electrically from each other will be described below.

[0066] The transparent, lattice constant transition layer 20 comprising
gradually increasing lattice constants is deposited and grown epitaxially
on the GaxIn1-xAs or GaxIn1-xAsyP1-y tunnel
junction layer 46 in order to make the transition from the lattice
constant of the InP substrate 26 and intervening layers described above
to a lattice constant that matches the GaxIn1-xAs or
GaxIn1-xAsyP1-y of the second subcell 24, which is
formulated to provide a desired bandgap Eg2, as described above.
According to one embodiment, the bandgap Eg2 is less than the
bandgap Eg1 of the first subcell 24 in the monofacial, inverted PV
converter embodiment 10 of FIGS. 1 and 2 for the reasons explained above.
InAsyP1-y is used for this lattice constant transition layer
20, because it can be formulated to lattice match the lower bandgap
Eg2 of the GaxIn1-xAs of
GaxIn1-xAsyP1-y of the second subcell 24, while it
also remains transparent to the longer infrared radiation R wavelengths
that are not absorbed by the higher bandgap Eg1 material of the
first subcell 22. This feature is important in order to ensure that
substantially all of the longer wavelength radiation R, which is not
absorbed in the first subcell 22, reaches the second subcell 24.

[0067] To form the lattice constant transition layer 20, (As) is added to
a growing layer of InP in increasing proportions so that the proportion
of arsenic (As) increases in the resulting InAsyP1-y material,
which increases the lattice constant of the InAsyP1-y. As
mentioned above, this change can be accomplished continuously, or the
changes in proportions be made in incremental steps. In the
InAsyP1-y of the lattice constant transition layer 20 of this
example PV converter 10, y varies from zero (where it lattice-matches the
Ga0.47In0.53As of the first subcell 22) to about 0.44, where it
lattice-matches to the GaxIn1-xAs of the second subcell 24, in
which x≈0.26 and the consequent bandgap Eg2 is about 0.55
eV. That example bandgap Eg2=0.55 eV enables the second subcell 24
to absorb infrared radiation R with wavelengths up to about 2.25 μm.
In general, the lattice-matching condition of GaxIn1-xAs to InAsyP1-y
occurs when the crystal lattices of the epi-layers are fully relaxed,
which is where y≈2.143x.

[0068] Of course, as mentioned above, the GaxIn1-xAs of the
second subcell 24 can have x equal to some other value for a different
desired bandgap Eg2, and the y in the InAsyP1-y of the
lattice constant transition layer 20 can be varied or customized
accordingly to make the necessary corresponding lattice constant
transition. Also, as mentioned above, either or both of the subcell
materials and/or the lattice constant transition materials could be
quaternary GaxIn1-xAsyP1-y with the x and y values
customized to desired bandgaps and lattice constants within the physical
constraints illustrated by the bandgap vs. lattice parameter chart of
FIG. 3.

[0069] As explained above and shown in FIG. 2, the second subcell 24
comprises a lattice-matched, double-heterostructure of n-type
InAsyP1-y front cladding layer 54, n-type GaxIn1-xAs
or GaxIn1-xAsyP1-y emitter layer 50 and p-type
GaxIn1-xAs or GaxIn1-xAsyP1-y base 52 to
form the homojunction 48, and p-type InAsyP1-y back cladding
layer 56, all grown epitaxially on the lattice constant transition layer
20. As explained above for the first subcell 22, the cladding layers 54,
56 confine and passivate the front and back surfaces of the homojunction
layers 50, 52 to prevent recombination of minority carriers. The
thicknesses and doping levels of the subcell 24 layers 50, 52, 54, 56 can
be similar to those described above for the first subcell 22.

[0070] A back surface spectral control element 28, which can also be used
as a back electrical contact layer, can be deposited onto the back
cladding layer 56 or onto an additional contacting layer (not shown)
disposed atop the back cladding layer 56. The nature of the back surface
spectral element 28 may depend on the application of the device 10. For
example, if the device 10 is a SPV or TVP, the sole purpose of which is
to convert radiation to electricity, then the back surface spectral
element may comprise a reflector to reflect any remaining, unabsorbed
radiation from the second subcell 24 back through the subcells 24, 22.
Some of such reflected radiation could be absorbed in this second pass
through the subcells, but most of it will continue propagating all the
way back through the substrate 26 toward whatever radiator source (not
shown) produces the incident radiation R in the first place. Adding such
unabsorbed, back-reflected, radiation energy back into the radiator
source may enable the radiator source to use such back-reflected energy
in the production of new incident radiation R for conversion to
electricity in the converter 10. This feature is particularly appropriate
for TPV configurations of converter 10 that are applied to convert
infrared radiation R produced by a blackbody infrared radiation source
(not shown) to electricity. Any radiation reflected back into the
blackbody radiator adds energy to the blackbody radiator and thereby
tends to raise the temperature of the blackbody radiator, which causes
the blackbody radiator to produce more blackbody infrared radiation for
the converter 10. Therefore, such back-reflected radiation can help the
blackbody radiator to produce more incident radiation R for the device 10
without having to use so much fuel.

[0071] On the other hand, some devices 10 are used both for producing
electricity and gathering heat for an environment. In those applications,
the back surface spectral control element 28 may be a material that is
transparent to remaining infrared radiation that is not absorbed by the
second subcell 24 so that such remaining infrared can be used as heat
someplace behind the device 10.

[0072] If the layer 28 is a back surface reflector (BSR), there can be
several advantages to designing the last (second) subcell 24 with only
one-half of its normal thickness, i.e., one-half the thickness that would
be required for full absorption of radiation in the wavelengths that
correspond to the bandgap, because any unabsorbed radiation will be
reflected by the BSR 28 back into the last subcell 24. The advantages of
this kind of design include an enhanced photocurrent, higher operating
voltage, and thinner structure that requires less growth time and
provides easier device processing. Regardless of its optical
characteristics, as described above, the layer 28 can also be a back
surface electrical contact. Therefore, it may be electrically conductive.
An optional, additional metallic contact 45 can also be used on the
conductive layer 28 for making an electrical connection, if desired.

[0073] The design of the front surface spectral control element 30 on the
front surface 27 of the substrate 26 may also depend on usage of the
device 10. For example, if the device 10 is to be used only for producing
electricity from blackbody radiation, the front surface spectral control
element 30 may be a coating layer that transmits only shorter wavelength
incident radiation R that can be absorbed and converted to electricity by
the subcells 22, 24 and that reflects all longer wavelength incident
radiation R back into the blackbody radiator (not shown) for recovery and
re-use. On the other hand, if the device 10 is to be used both for
producing electricity and heat for an environment, then the front surface
spectral control element 30 may be an antireflective coating to enhance
transmission of all the incident radiation R into the device 10.

[0074] As mentioned above, the monofacial PV converter 10 described above
and illustrated in FIGS. 1 and 2 is configured with the subcells 22, 24
connected electrically in series facilitated by the tunnel junction
layers 44, 46. An alternate embodiment monofacial, low-bandgap,
monolithic, multi-bandgap, PV converter 110 is shown in FIG. 4 with much
the same first subcell 22, second subcell 24, and substrate 26 structures
and materials described above for the PV converter 10, but with the
subcells 22, 24 isolated electrically from each other. The electrical
isolation instrumentality in the PV converter 110 is illustrated as a
discrete electrical isolation layer 39 positioned between the first
subcell 22 and the second subcell 24. However, such electrical isolation
function could be incorporated into other components, such as into the
graded lattice constant transition layer 20, as will be explained below.

[0075] There are a number of reasons that such electrical isolation of the
subcells 22, 24 may be desirable in some applications. For example, as
mentioned above, current flow through series connected subcells 22, 24 is
limited by the lowest photocurrent producing subcell. Therefore, for
series connected subcells, a number of subcell design factors, such as
bandgaps, thicknesses, doping concentrations, and the like are used to
optimize the operating characteristics of the series connected subcells
22, 24, so that electric power production from the tandem combination is
maximized. In some designs and applications, however, more efficient
conversion of radiant energy to electricity can be accomplished by
extracting electric power from the individual subcells 22, 24 separately
or independently, or, in some applications, to design the subcells 22, 24
for voltage matching. Such voltage matching techniques with subcells in
other devices will be discussed in more detail below in relation to
monolithic, integrated module (MIM) devices.

[0076] To isolate the subcells 22, 24 electrically from each other, there
has to be some material between them that inhibits electric current flow
between the subcells 22, 24. However, such electrical isolation material
cannot interfere with radiation transmission from one subcell 22 to the
other subcell 24. In the PV converter 110 of FIG. 4, a discrete isolation
layer 39 is shown positioned between the first subcell 22 and the lattice
constant transition layer 20, although it could be positioned between the
lattice constant transition layer 20 and the second subcell 24.

[0077] An isolation material for isolation layer 39 can be fabricated in a
number of ways. One such approach is to fabricate the isolation layer 39
with a high-resistivity semiconductor material that has a high enough
bandgap to be transparent to the longer wavelength radiation that is not
absorbed in the first subcell 22 and is being transmitted to the second
subcell 24. Another such approach is to form the isolation layer 39 as an
isolation diode, which, of course, may also be transparent to the
radiation being transmitted from the first subcell 22 to the second
subcell 24. Also, such high-resistivity material or isolation diode
material has to be lattice-matched to the materials in front and in back
of it, which, in the position of isolation layer 39 shown in FIG. 4, has
to be lattice-matched to the first cell 22 and substrate 26.

[0078] As mentioned above, InP doped with a deep acceptor element, such as
Fe or Cr, is a high-resistivity material and has a bandgap (1.35 eV) that
makes it transparent to all radiation that is not absorbed by the first
subcell 22. It is also lattice-matched to the InP substrate 26 and to the
ternary Ga0.47In0.53As or quaternary
GaxIn1-xAsyP1-y of the first subcell 22. Therefore,
deep acceptor-doped InP can be used as the high-resistivity, isolation
layer 39. Such deep acceptor-doping of other lattice-matched
semiconductor materials, such as ternary GaxIn1-xAs, quaternary
GaxIn1-xAsyP1-y, or even AlGaInAs in some
circumstances, with high enough bandgaps to be transparent to the
radiation being transmitted, could also be used to provide suitable
high-resistivity materials for the isolation layer 39.

[0079] An isolation diode for isolation layer 39 can be provided by one or
more doped junctions, such as an n-p junction or n-p-n junctions with
high enough reverse-bias breakdown characteristics to prevent current
flow between the subcells 22, 24. Again, lattice-matched semiconductor
materials, such as InP, GaxIn1-xAs or
GaxIn1-xAsyP1-y, or even AlGaInAs, can be doped to
provide an isolation diode structure for isolation layer 39.

[0080] While a discrete isolation layer 39 is shown in the PV converter
110 of FIG. 4, it is possible to dope the lattice constant transition
layer 20 to also function as an isolation layer between the two subcells
22, 24. The InAsyP1-y of the lattice constant transition layer
20 can also be doped with a deep acceptor element, such as Fe or Cr, to
make it highly-resistive, or discrete sublayers of the
InAsyP1-y can be n-p or n-p-n doped to form an isolation diode
structure.

[0081] Of course, with each subcell 22, 24 isolated electrically from each
other, some additional provisions for electrical contacts are necessary
to extract electric power independently from each subcell 22, 24. Persons
skilled in the art will be able to design myriad structures for such
contacts, once they understand the principles described in this
disclosure. The example additional contacts 27, 42 for this purpose are
shown fabricated on lateral current flow layers 39, 41 respectively. Such
lateral current flow layers 39, 41 are lattice-matched to their
respective subcells 22, 24 and should be transparent to radiation being
transmitted from the first subcell 22 to the second subcell 24. Heavily
doped GaIn1-xAsyP1-y with 0≦x≦1 and
0≦y≦1 as necessary for lattice matching and transparency
can be used for these lateral current flow layers 39, 41.

[0082] While the series connected PV converter 10 and isolated or
independently connected PV converter 110 described above are illustrated
with only two subcells 22, 24, and only one lattice constant transition
layer 20 between them, any number of subcells with any number of lattice
constant transition layers can be included in a monolithic,
multi-bandgap, optoelectronic device according to alternate embodiments.
To illustrate this principle, a more complex monolithic, multi-bandgap,
PV converter 112 is illustrated in FIG. 5.

[0083] In the PV converter 112, an arbitrary number (five) subcells 114,
116, 118, 120, 122 are illustrated with arbitrary bandgaps
Eg1>Eg2>Eg3>Eg4>Eg5. The substrate
124 is InP, and the first and second subcells 114, 116 both have bandgaps
Eg1, Eg2 that can be ternary GaInAs or quaternary GaInAsP and
are lattice-matched to the InP substrate 124 (see, e.g., lines 130, 12,
14 in FIG. 3). These first and second subcells 114, 116 may be both
lattice-matched (LM), double-heterostructures (DH) with junctions
comprising n-type and p-type ternary GaxIn1-xAs or quaternary
GaxIn1-xAsyP1-y clad with n-type and p-type layers of
InP, as described above for the other PV converter embodiment 10. The
third and fourth subcells 118, 120 are LM, DH ternary or quaternary
GaInAs(P) with bandgaps Eg3, Eg4 that can be lattice-matched to
each other, but not to the InP substrate (see, e.g., lines 12, 132, 134,
136 in FIG. 3). Therefore, a lattice constant transition layer 126 of
graded InAsyP1-y is used to make the transition from the second
subcell 116 to the lattice-mismatched subcell 118. A fifth LM, DH subcell
122 of ternary or quaternary GaInAs(P) has a bandgap Eg5 that cannot
be lattice-matched to the fourth subcell 120. Therefore, another lattice
constant transition layer 128 is provided to make the transition from the
fourth subcell 120 to the lattice-mismatched fifth subcell 122.

[0084] As mentioned above, the numbers and combinations of subcells and
lattice constant transition layers as well as the specific example
bandgap values shown in the PV converter 112 of FIG. 5 are selected
arbitrarily for illustrative purposes. The only requirement is that the
incident radiation reaches the subcells in order of decreasing bandgaps,
so that the shorter wavelength radiation is absorbed and converted to
electricity by higher bandgap subcells that will transmit unabsorbed,
longer wavelength radiation to the next subcell(s). Other details, such
as buffer layers, tunnel junction or isolation layers, contacts, optic
control layers, etc., for fabricating a working PV converter can be
similar to those described above for either the series connected subcell
embodiments 10 of FIGS. 1 and 2 or the independently connected subcell
embodiment 110 of FIG. 4.

[0085] Now, as illustrated in another alternative inverted, monofacial,
multi-bandgap, PV converter 140 in FIG. 6, the positions of the
transparent lattice constant transition layer 20 and the first subcell 22
positions can be reversed from their positions shown in the FIG. 1
embodiment 10. Specifically, the lattice constant transition layer 20 can
be grown expitaxially on the InP substrate 26 by gradually adding more
and more As to the growing InAsyP1-y lattice constant
transition layer 20, as described above, until a desired lattice constant
is attained for a desired GaxIn1-xAs or
GaxIn1-xAsyP1-y semiconductor material with a desired
bandgap to be grown on the InP substrate 26. As explained above, the
desire bandgap is chosen for absorbing and converting infrared radiation
R of a desired wavelength or frequency band to electricity.

[0086] For example, but not for limitation, if it is desired to have the
first subcell 22 in the PV converter 140 of FIG. 6 absorb and convert
infrared radiation of at most 1.77 μm wavelength to electricity and to
have the second subcell 24 absorb and convert infrared radiation in the
range of 1.77 μm to 2.14 μm to electricity, the first subcell 22
would need a bandgap of about 0.70 eV, and the second subcell 24 would
need a bandgap of about 0.58 eV. Therefore, an appropriate lattice
constant transition layer 20 can be InAsyP1-y with a gradually
increasing proportion of As until an InAsyP1-y semiconductor
material having a lattice constant of about 5.94 Å and a bandgap of
about 0.90 eV, as illustrated in FIG. 7 by broken lines 60, 62,
respectively. Therefore, a lattice constant transition layer 20 with
those criteria will provide a transition of lattice constant 12 from the
5.87 Å of the InP substrate to the 5.94 Å of the terminal
InAsyP1-y material in the lattice constant transition layer 20.
With a terminal bandgap of 0.90 eV, the lattice constant transition layer
20 is transparent to infrared radiation (IR) with wavelengths longer than
about 1.38 μm.

[0087] The first subcell 22 with the example desired 0.70 eV bandgap can
then be a lattice-matched (LM), double-heterostructure (DH) of, for
example, InAsyP1-y/GaxIn1-xAsyP1-y with the
same lattice constant, 5.94 Å, as the terminal InAsyP1-y of
the lattice constant transition layer 20 (see broken line 60 in FIG. 7).
The GaxIn1-xAsyP1-y base of the subcell 22 can be
formulated to have the desired bandgap of 0.70 eV (see broken line 64 in
FIG. 7 and corresponding line 64 in FIG. 6), so it will absorb and
convert 1.77 μm and shorter radiation R to electricity, but it will
transmit and not absorb virtually all the incident infrared radiation
(IR) that is longer wavelength than 1.77 μm. Such formulation of
appropriate proportions of Group III-V elements in quaternary alloys,
such as the GaxIn1-xAsyP1-y in this example, to
achieve certain desired bandgap characteristics, such as the 0.70 eV in
this example, is well-known and within the capabilities of persons
skilled in the art, thus need not be explained in detail here to enable
persons skilled in the art to understand and practice these embodiments.

[0088] The second subcell 24 in this example can be formulated with a
lattice-matched (LM), double-heterostructure (DH) with the same lattice
constant of 5.94 Å as the first subcell 22 and still have a bandgap
as low as 0.58 eV (see broken lines 60, 66 in FIG. 7). Therefore, if it
is desired to formulate the second cell 24 to absorb and convert as much
of the infrared radiation R, which passed through the first cell 22, as
possible, and still be lattice-matched to the first cell 22, then
GaxIn1-xAs with a bandgap of 0.58 eV can be used. This example
second cell 24, with its 0.58 eV bandgap 66, would absorb and convert
infrared radiation R of 2.14 μm wavelength and shorter to electricity.
Of course, other auxiliary layers, such as buffers, cladding, tunnel
junction or isolation layers, contacts, and antireflective or optical
control layers can be used to make this structure a functioning device,
as explained above, and as would be understood by persons skilled in the
art.

[0089] While two lattice-mismatched (LMM) subcells 22, 24 and one lattice
constant transition layer 20 in any of a variety of ternary and/or
quaternary formulations comprising Ga, In, As, and/or P provide wide
flexibility in low bandgap designs for efficient absorption and
conversion of desired infrared radiation wavelength bands to electricity,
embodiments also extends to three, four, five, or more subcells and
bandgaps with one or more lattice constant transition layers, as needed.
For example, there is no theoretical limit to the number of quaternary
GaxIn1-xAsyP1-y formulations for different bandgaps
between lines 13 and 14 (0.74 eV to 1.35 eV) in FIG. 3, which can be
lattice-matched on line 12 (5.869 Å). Likewise, there is no
theoretical limit to the number of quaternary
GaxIn1-xAsyP1-y formulations for different bandgaps
between lines 14 and 17 (0.74 eV to 0.52 eV) in FIG. 3, which can be
lattice-matched on line 15 (5.968 Å). Further, there is no
theoretical limit to the number of ternary and quaternary Ga, In, As,
and/or P formulations for possible lattice constants between those of InP
(5.869 Å) and InAs (6.059 Å), lines 12 and 23, respectively, in
FIG. 3.

[0090] In other words, every ternary GaxIn1-xAs or quaternary
GaxIn1-xAsyP1-y with a bandgap in the range between
0.74 eV and 0.355 eV (lines 14 and 31 in FIG. 3) can be lattice-matched
to some higher bandgap InAsyP1-y, which is transparent to at
least some infrared radiation that can be absorbed and converted to
electricity by such ternary GaxIn1-xAs or quaternary
GaxIn1-xAsyP1-yDH subcells. Further, any
InAsyP1-y, which is used to make a transition between the
lattice constant of such ternary GaxIn1-xAs or quaternary
GaxIn1-xAsyP1-y to a larger lattice constant, also
has a higher bandgap than such GaxIn1-xAs or
GaxIn1-xAsyP1-y, thus is transparent to at least some
infrared radiation that can be absorbed and converted to electricity by
such ternary GaxIn1-xAs or quaternary
GaxIn1-xAsyP1-y DH subcells. One or more embodiments
described herein utilize these characteristics of
GaxIn1-xAsyP1-y (0≦x≦1,
0≦y≦1) for the design, formulation, and fabrication of low
bandgap (less than 1.35 eV, and may be less than 0.74 eV), monolithic,
multi-bandgap, photovoltaic converters, as described above, and as will
be further described below.

[0091] Embodiments describer by this disclosure, as mentioned above, also
extends to low bandgap, monolithic, multi-bandgap PV converters with more
than one lattice constant transition layer. For example, referring again
to FIG. 6, one or more additional subcells with even lower bandgap(s)
than the 0.58 eV bandgap of the second subcell 24 can be grown on top of
subcell 24. Such an example PV converter 150 with three subcells 22, 24,
72 is illustrated diagrammatically in FIG. 8. This example three-bandgap
PV converter 150 is illustrated for convenience with the same substrate
26, first lattice constant transition layer 20, first subcell 22, and
second subcell 24 as the two-bandgap embodiment 140 of FIG. 6, but it has
a second lattice constant transition layer 70 positioned between the
second subcell 24 and a third subcell 72.

[0092] As was explained above in relation to the inverted tandem
(two-subcell) PV converter 140 in FIG. 6, the InP substrate 26 and the
first lattice constant transition layer 20 are transparent to infrared
radiation of longer wavelengths than can be absorbed by their respective
bandgap characteristics. Therefore, in the examples of FIGS. 6 and 8, the
lowest bandgap of the InAsyP1-y lattice constant transition
layer 20 is 0.90 eV (see line 62 in FIG. 7), which, of course, is also
lower than the 1.35 eV bandgap of the InP substrate. Therefore, infrared
radiation of wavelengths longer than 1.38 μm pass through both the InP
substrate 26 and the InAsyP1-y lattice constant transition
layer 20. The first subcell 22, with its 0.70 eV bandgap, absorbs and
converts infrared radiation R wavelengths of 1.77 μm and shorter to
electricity, and it transmits infrared radiation R wavelengths longer
than 1.77 μm to the second subcell 24. The 0.58 eV bandgap of the
GaxIn1-xAs second subcell 24 enables it to absorb and convert
infrared radiation wavelengths of 2.14 μm and shorter to electricity,
while infrared radiation R wavelengths greater than 2.14 μm pass
through the second subcell 24.

[0093] The energy in the infrared radiation R wavelengths longer than 2.14
μm, which are not absorbed in the second subcell 24 would be wasted in
the PV converter 140 embodiment of FIG. 6, but adding one or more
additional subcells, such as subcell 72 in the three-bandgap PV converter
150 in FIG. 8, can capture and convert significant amounts of that energy
to electricity. However, as shown by line 60 in FIG. 7, there is no
ternary GaxIn1-xAs or quaternary
GaxIn1-xAsyP1-y with a bandgap below 0.58 eV (line
66) that has the same lattice constant (5.94 Å) as the first and
second subcells 22, 24 in the example. Therefore, a second lattice
constant transition layer 70 comprising InAsyP1-y with
gradually increasing proportions of arsenic (As) is positioned between
the second subcell 24 and the third subcell 72. The initial
InAsyP1-y in the second lattice constant transition layer 70 is
formulated to have a bandgap of 0.90 eV, so that it has the same lattice
constant (5.94 Å) as the second subcell 24. Then, the subsequent
InAsyP1-y grown for the second lattice constant transition
layer 70 decreases in bandgap in incremental steps or gradually toward
the same bandgap 66 as the second subcell 24, which is 0.58 eV in the
example described above. At that bandgap level, the InAsyP1-y
is still transparent to all of the infrared radiation R wavelengths that
pass through the second subcell 24. Therefore, the InAsyP1-y
second lattice constant transition layer 70 does not absorb or interfere
with the infrared radiation R that has to reach the third subcell 72, yet
it provides a transition from the lattice constant of the second subcell
24 (line 60 in FIG. 7) to a new, larger lattice constant (line 74 in FIG.
7) for the third subcell 72. In this example, the new lattice constant of
6.02 Å will match a ternary GaxIn1-xAs with a bandgap of
0.45 eV or quaternary GaxIn1-xAsyP1-y with a bandgap
anywhere along line 94 between 0.58 eV and 0.45 eV, as shown in FIG. 7.
Therefore, if the third subcell 74 in FIG. 8 comprises, for example,
GaxIn1-xAs formulated to have a bandgap of 0.45 eV, it will
absorb and convert infrared radiation in wavelengths of 2.76 μm and
shorter to electricity.

[0094] Again, as explained above for the first lattice constant transition
layer 20, the gradual change of lattice constant in the second lattice
constant transition layer 70 can be graded gradually or in discrete
stepped increments. Also, while not shown in detail in FIG. 8, the
GaxIn1-xAs n/p junction in the third subcell 72 may be clad
front and back with cladding layers of InAsyP1-y, which have
the same lattice constant as the GaxIn1-xAs of the third
subcell 74, to form the third subcell 72 as a lattice-matched (LM),
double-heterostructure (DH) subcell. Other auxiliary layers mentioned
above can also be provided.

[0095] All of the PV converter embodiments 10, 110, 140, 150 described
above have been monofacial, i.e., grown on only one face of the
substrate. A significant feature of one or more embodiment is that they
can also be implemented in bifacial or buried substrate structures, as
illustrated diagrammatically by the example low bandgap, monolithic,
multi-bandgap, PV converter 80 in FIG. 9. Essentially, in the PV
converter 80 of FIG. 9, a lattice-matched (LM) first subcell 82 is grown
epitaxially on a front surface 83 of a InP substrate 84, and a
lattice-mismatched (LMM) second subcell 86 with an intervening lattice
constant transition layer 90 is grown epitaxially on a back surface 85 of
the substrate 84. An antireflective coating (ARC) 88 on the front surface
81 of the first subcell 82 and a back surface reflector (BSR) 89 on the
back surface 87 of the second subcell 86 are shown, but they can be other
optical control layer materials, as described above for PV converter 10.
Again, other auxiliary features, layers, and components that may be used
to implement an actual device, such as buffers, contacts, deep acceptor
doping of the InP substrate, tunnel junctions or isolation layers, and
the like are not shown separately in FIG. 9 in order to avoid unnecessary
clutter and repetition, but persons skilled in the art can use the
information herein to understand, design, and fabricate such components
in PV converter devices according to embodiment described herein.
Cladding layers (not shown separately in FIG. 9) can be used as part of
the subcells 82, 86 for lattice-matched (LM), double-heterostructure (DH)
implementations of the subcells, as described above in relation to the PV
converter 10. Also, while only one lattice constant transition layer 90
and two subcells 82, 86 with specific example bandgaps and lattice
constants are illustrated in the example PV converter 80 in FIG. 9, other
numbers of subcells, lattice constant transition layers, bandgaps, and/or
lattice constants can also be used, as explained above.

[0096] In the example bifacial PV converter 80 in FIG. 9, the first
subcell 82 is lattice-matched to the InP substrate 84 (line 12 in FIG.
10). In this example, subcell 82 is formulated to have the lowest
possible bandgap that can be lattice-matched to the InP substrate 84,
which is the ternary Ga0.47In0.53As with a bandgap of 0.74 eV
(line 14 in FIG. 8), although many other formulations could be
illustrated, as explained above. If it is desired to use a
lattice-matched, double-heterostructure for the first subcell 82, the
Ga0.47In0.53As n/p junction material can be clad on both sides
with lattice-matched, epitaxially grown, InP cladding layers, as
described above.

[0097] In this example, any incident radiation R of wavelengths shorter
than 1.67 μm will be absorbed by the 0.74 eV bandgap
Ga0.47In0.53As in the first subcell 82, and longer wavelength
infrared radiation R will pass through the first subcell 82. The InP
substrate 84, which has a much higher bandgap of 1.35 eV (line 13 in FIG.
10) is also transparent to any of such longer wavelength infrared
radiation that passes through the first subcell 82. Therefore, in order
for the second subcell 86 to absorb and convert any of such longer
wavelength infrared radiation R to electricity, it has to have a bandgap
Eg2 that is less than the bandgap Eg1 of the first subcell 82,
i.e., less than 0.74 eV in this example. There are many considerations
for selecting the lower bandgap for the second subcell 86, such as
targeting the concentrations of the infrared radiation in various
wavelength or frequency bands, conversion efficiencies, and any
additional subcells (not shown in FIG. 9). However, any bandgap less than
0.74 eV requires a GaxIn1-yAs or
GaxIn1-xAsyP1-y that has a larger lattice constant
than the InP substrate 84, so it would not be possible for it to be
lattice-matched to the InP substrate 84. Therefore, in such an
embodiment, a lattice constant transition layer 90 may be needed, and the
second subcell 86 may have a lattice constant that allows the
InAsyP1-y lattice constant transition layer 90 to be
transparent to the longer infrared radiation wavelengths, which are not
absorbed by, and pass through, the first subcell 82. In this example, a
bandgap Eg2 for the second subcell 86 is selected to be 0.55 eV,
which can be provided with ternary GaxIn1-xAs having a lattice
constant of 5.972 Å, as illustrated by lines 92 and 93 in FIG. 10.
However, a quaternary GaxIn1-xAsyP1-y with a slightly
larger lattice constant could also be used for the same bandgap and still
be able to accommodate a transparent lattice constant transition layer
90.

[0098] As shown by lines 12, 93 in FIG. 10, the ternary
GaxIn1-xAs with its lattice constant of 5.972 Å requires
the lattice constant transition layer 90 to make the transition between
the lattice constant of the InP substrate (line 12) to the 5.972 Å
lattice constant (line 93). As also illustrated in FIG. 10, increasing
the proportion of As in an InAsyP1-y lattice constant
transition layer 90 reaches this 5.972 Å constant without its bandgap
ever decreasing below about 0.82 eV (line 94), which is still higher than
the 0.74 eV bandgap (line 14) of the first subcell 82. Therefore, any
infrared radiation R that is not absorbed by, thus passes through, the
first subcell 82 will also not be absorbed by the InAsyP1-y in
the lattice constant transition layer 90. Consequently, such infrared
radiation will reach the second subcell 86, where at least some of it can
be absorbed and converted to electricity.

[0099] If the substrate 84 is doped with a deep acceptor element, such as
Fe or Cr, to be an insulator or semi-insulator, as explained above, then
the first subcell 82 and the second subcell 86 are electrically isolated
from each other. Therefore, electricity has to be extracted independently
from each subcell 82, 86, as described above for the electrically
isolated subcells 22, 24 of the PV converter device 110 in FIG. 4. This
feature has advantages, such as in voltage-matching of multiple, series
and/or parallel interconnected PV converter subcell circuits, especially
in monolithic, integrated module (MIM) devices, as will be described in
more detail below. In situations where the substrate 86 cannot be made as
an insulator or semi-insulator, a separate isolation layer (not shown in
FIG. 9) can be positioned anyplace between the two subcells 82, 86. For
example, an isolation layer can be grown on either the front surface 83
or back surface 85 of the substrate 84 or between the lattice constant
transition layer 90 and the second subcell 86. Such an isolation layer
can be made as desired above in relation to the isolation layer 39 in
FIG. 4, i.e., a lattice-matched material that is transparent to
wavelengths of radiation not absorbed by the first subcell 82 and doped
to make the material highly-resistive or to create a diode barrier to the
flow of electric current. Of course, there could also be applications
that involve a series connection of subcell 82 on the front side of the
substrate 84 with the subcell 86 on the back side of the substrate 84, in
which case the substrate 84 should be doped to conduct current, and
appropriate tunnel junction layers may be added to allow current flow as
explained above in relation to the PV converter device 10 of FIG. 2.

[0100] As explained above, any of a wide range of ternary or quaternary
GaInAs(P) alloys with any combinations of bandgaps and lattice constants
can be used in subcells of tandem (more than one subcell) stacks of
low-bandgap, monolithic, multi-bandgap, optoelectronic devices. Another
example of such combinations is illustrated in the alternate example
bifacial PV converter device 160 in FIG. 11, where two lattice mismatched
(LMM) subcells 162, 164 are grown on the front side 165 of an InP
substrate 166 and another, even lower bandgap, subcell 168 is grown on
the back side 167 of the substrate 166. For purposes of this
illustration, but not for limitation, the first subcell 162 is shown as
LM, DH, quaternary GaxIn1-xAsyP1-y with a bandgap of
1.1 eV, but lattice-matched to a LM, DH, ternary GaxIn1-xAs
second subcell 164 instead of to the InP substrate 166, as shown by lines
170, 172, 174 in FIG. 10. Therefore, a lattice constant transition layer
176, such as graded InAsyP1-y is needed between the InP
substrate 166 and the second subcell 164, as shown in FIG. 11, to make
the transition between the 5.869 Å lattice constant of the InP
substrate 166 (line 12 in FIG. 10) and the 5.905 Å lattice constant
of the ternary GaxIn1-xAs of the second subcell 164 (line 174
in FIG. 10). Either an isolation layer 178, as shown in FIG. 11, or a
tunnel junction, can be positioned between the first subcell 162 and the
second subcell 164, depending on whether it is desired to connect the
subcells 162, 164 independently or in series, as explained above.

[0101] The third subcell 168 in the PV converter 160 in FIG. 11 has an
even lower bandgap, for example 0.55 eV, to absorb and convert longer
wavelength radiation transmitted through the first and second subcells
162, 164 to electricity, according to the principles explained above.
Such a third subcell 168 can be, for example, a LM, DH, ternary
GaxIn1-xAs with a 0.55 eV bandgap, which is not lattice-matched
to the InP substrate 166, as shown by lines 12, 93 in FIG. 10. Therefore,
another lattice constant transition layer 180, such as graded
InAsyP1-y is needed between the InP substrate 166 and the third
subcell 168 to make the transition between the 5.869 Ålattice
constant of InP (line 12 in FIG. 10) and the 5.952 Å lattice constant
of the ternary GaxIn1-xAs (line 93 in FIG. 10) of the third
subcell 168.

[0102] Again, if the InP substrate 166 is deep acceptor doped to be an
insulator or semi-insulator, the third subcell 168 will be electrically
isolated from the first and second subcells 162, 164 and can be connected
independently to other PV converters or subcells, such as in a MIM
structure (described below). Otherwise, a separate isolation layer (not
shown in FIG. 11) may be needed somewhere between the substrate 166 and
the third subcell 168, as explained above. Again, contacts, conductive
layers, buffers, optical control layers, and the like, are not shown in
FIG. 11, but can be provided as explained above for other embodiments.

[0103] Another interesting variation of the bifacial embodiment PV
converter 80 in FIG. 9 is the use of epitaxially grown InP for the higher
bandgap first subcell 82 instead of a ternary or quaternary GaInAs(P).
This variation, with an appropriate lower bandgap (lower than the 1.35 eV
bandgap of InP) second subcell 86, can operate as a highly efficient,
stand-alone, tandem solar cell. This bifacial or buried substrate
configuration of the PV converter 80 is particularly advantageous for use
as a solar cell, because the buried InP substrate 84 is not in a position
to block or absorb shorter wavelength solar radiation before it reaches
the first subcell 82.

[0104] An illustration of this principle in a slightly more complex
bifacial, monolithic, multi-bandgap, solar photovoltaic (SPV) converter
device 190, multiples of which can also be incorporated into a MIM
structure, is shown in FIG. 12. In this example SPV device 190, there are
three lattice-matched subcells 192, 194, 196 grown on the front side 197
of a InP substrate 198 and two, lower bandgap, lattice-mismatched (LMM)
subcells 200, 202 grown on the back side 199 of the substrate 198. Again,
isolation and/or tunnel junction layers 204, 206 can be included between
front-side subcells 192, 194 and/or between subcells 194, 196,
respectively, for either independent electrical connection or series
electrical connection, respectively, within the SPV device 190, as
explained above. Similarly, either an isolation layer 208 or a tunnel
junction can be provided between the back-side subcells 200, 202,
depending on whether it is desired to electrically connect them
independently or in series within the SPV device 190.

[0105] In the example SPV device 190, the first subcell 192 is shown as a
LM, DH, InP subcell with a bandgap of 1.35 eV, while the bandgaps of the
second and third subcells 194, 196 have lower bandgaps, e.g., 1.0 eV, LM,
DH, quaternary GaxIn1-xAsyP1-y for the second subcell
194 and 0.74 eV, LM, DH, ternary Ga0.47In0.53As for the third
subcell 196, which is the lowest bandgap GaInAs that can be
lattice-matched to the InP substrate 198 (see lines 13, 14, 210 in FIG.
13). Since all the front-side subcells 192, 194, 196 are lattice-matched
(line 12 in FIG. 13) to the InP substrate 198, no lattice constant
transition layer is needed on the front side of the substrate 198.

[0106] Again, the variations of conductive or highly-resistive substrate
198, isolation and/or tunnel junction layers, electrical contacts, buffer
layers, optical control layers, more or fewer subcells, different
bandgaps, and the like, as described above for other embodiments, are
also applicable to the SPV device 190 described above.

[0107] Also, AlInAs in slightly higher bandgaps than the 1.35 eV bandgap
of the InP can also be lattice-matched to InP, so a first subcell of such
AlInAs lattice-matched to the InP substrate could also be used as part of
a bifacial, monolithic, multi-bandgap, PV converter. Of course, Ga could
be added to produce AlGaInAs, if a slightly lower bandgap than AlInAs may
be desired for either the first subcell or a subsequent subcell of a
bifacial PV converter.

[0108] The PV converters described above can be used alone or in
combinations with myriad other devices. For example, any of the PV
converters, especially the SPV device 190, but also, PV converters 10,
110, 112, 140, 150, 80, 160, can be used for the bottom cell device in a
mechanical stack of higher bandgap (higher than the 1.35 eV bandgap of
InP) PV converters, such as GaAs based PV converters, in solar cell and
other applications. Such other, higher bandgap, PV converters (not shown)
can selectively absorb and convert shorter wavelength solar energy to
electricity, while the lower bandgap PV converters, e.g., PV converters
10, 110, 112, 150, 80, 160, 190, absorb and convert longer wavelength
solar radiation to electricity.

[0109] As mentioned above, one or more of the bifacial, monolithic,
multi-bandgap, optoelectronic devices described herein, for example, the
bifacial PV converters 80, 160, 190 shown in FIGS. 9, 11, and 12 and
described above, as well as myriad variations of such bifacial
configurations, are particularly adaptable to use in monolithic,
integrated modules (MIMs). An example MIM PV converter device 230 with a
plurality of bifacial, monolithic, multi-bandgap, photovoltaic converters
190 of FIG. 12 grown on a single substrate 198 is shown in FIG. 14.
Essentially, all of the PV converter subcell stacks 190 are grown in
unison on the common substrate 198, and then they are separated into a
plurality of individual subcell stacks 190' by etching away or otherwise
removing material to form isolation trenches 232 between the front-side
subcell stacks 190' and to form isolation trenches 234 between the
back-side subcell stacks 190''. Then, various conductors 236, 238, 240,
242, 244, 246, 248, 250, 252, 254, 256, 258, 260, and others are added in
various electrical connection patterns to interconnect the subcells
together, as will be described in more detail below. The spaces between
the conductors are filled with insulator material, such as silicon
nitride or any of a variety of other suitable insulator materials.

[0110] In the bifacial MIM PV converter device 230 illustrated in FIG. 14,
there are twice as many, albeit smaller, back-side subcell stacks 190''
as front-face subcell stacks 190'. Further, there can be any desired
ratio of back-side subcell stacks 190'' to front-side subcell stacks
190'. The ratio of two back-side subcell stacks 190'' to one front-side
subcell stack 190' shown in FIG. 14 is only an example.

[0111] One advantage of being able to have different numbers of subcell
stacks 190', 190'' on front and back of the substrate 198 is more
flexibility to design voltage-matched subcell circuits. A schematic
diagram of an equivalent electrical circuit corresponding to the example
voltage-matched subcell circuits 262, 264, 266, 270, 272 of the MIM 230
of FIG. 14 is shown in FIG. 15. according to fundamental electrical
principles, a circuit comprising a plurality of subcells connected in
parallel will have an output voltage equal to the subcell output voltage,
but current from the parallel connected subcells add. Conversely, a
circuit comprising a plurality of subcells connected in series will have
a current output equal to the subcell output current, but the output
voltages of the series connected subcells add. Therefore, connecting a
plurality of higher voltage subcells together in parallel can build
current as output voltage remains constant, while connecting a plurality
of lower voltage subcells together in series can boost the voltage output
of the subcell circuit to the level of the higher voltage subcell
circuit. Also, higher bandgap subcells produce higher voltage than lower
bandgap subcells. Therefore, if, for example, the voltage output of each
of the lower bandgap subcell stacks 190'' in FIG. 14 is half as much as
the voltage output of each of the higher bandgap subcell stacks 190', and
if all the higher bandgap subcell stacks 190' are connected in series
with each other while all the lower bandgap subcell stacks 190'' are
connected in series with each other, then the total voltage output of the
back-side subcell stacks 190'' circuits 270, 272 would equal the total
voltage output of the front-side subcell stacks 190' circuits 262, 264,
266, because there are twice as many low voltage subcell stacks 190'' as
there are higher voltage subcell stacks 190'.

[0112] However, the front-side subcells 192, 194, 196 of the front-side
stacks 190' can be connected in myriad combinations of series and/or
parallel electrical connections, as illustrated in FIGS. 14 and 15 to
create voltage-matched subcell circuits 262, 264, 266. The same goes for
the back-side subcells 200, 202 of the back-side stacks 190'' to create
voltage-matched subcell circuits 270, 272. Such electrical connection
options are facilitated by the isolation layers 204, 206, 208 and
highly-resistive substrate 198, as described above. Additional options
can be provided by tunnel junctions instead of isolation layers or even
making the substrate 198 conductive rather than resistive for
intra-subcell stack series connections, as explained above.

[0113] To illustrate several series and parallel connection options, the
bifacial MIM PV converter device 230 in FIGS. 14 and 16, is shown, for
example, with all of its highest bandgap, thus highest voltage, subcells
192 connected together in parallel to form the subcell circuit 262. The
next highest bandgap, thus next highest voltage, subcells 194 are
connected together in a combination of parallel 244, 246 and series 247
connections to form a subcell circuit 264 that is voltage-matched to the
subcell circuit 262. The subcells 196, which are the lowest bandgap, thus
lowest voltage, of the subcells on the front side of the substrate 198,
are shown in this example illustration of FIGS. 14 and 15, as all being
connected in series in subcell circuit 266 by conductors 242 to add their
voltages in order to match the output voltage of subcell circuit 266 to
the output voltages of the subcell circuits 262, 264. These parallel and
series-connected subcell circuits 262, 264, 266 are connected in parallel
to each other at conductors 236, 238, 240 and at 236', 238', 240' to add
their respective current outputs.

[0114] The back-side subcells 200, 202 are even lower voltage than the
front-side subcells 196, but there are more of them than the front-side
subcells 192, 194, 196, so the back-side voltage can be matched to the
front-side voltage. In the example of FIGS. 14 and 15, the lowest voltage
subcells 202 are connected together in series in the subcell circuit 272
by conductors 260 to add their voltages in order to match the output
voltage of subcell circuit 272 to the output voltage of the parallel
connected 256, 258, higher voltage subcells 200 in the subcell circuit
270. Then, the subcell circuit 272 is connected in parallel to the
subcell circuit 270 by conductors 252, 254 and 252', 254' to add their
respective current outputs.

[0115] Finally, the front-side subcell circuits 262, 264, 266 are
connected in parallel to the back-side subcell circuits 270, 272 at
terminal contacts 258, 258' to add their respective current outputs.
Therefore, the bifacial MIM PV converter 230 can be connected
electrically to other devices or loads via the two terminal contacts 256,
258, which may be a desirable feature. Other MIM structures, circuit
connections, and advantages can be made according to these principles
within this scope of embodiments. For example, but not for limitation,
the monofacial, monolithic, LM, DH, multi-bandgap, PV converters
described above can also be incorporated into MIM structures (not shown),
although the bifacial embodiments described above have the advantage of
using the substrate 198 as a built-in isolation structure between
subcells on the front side and subcells on the back side, as explained
above.

[0116] Any of the PV converter embodiments 10, 110, 150, described above
and shown in FIGS. 1, 2, 4, and 6 can be modified to provide an
ultra-thin, monolithic, multi-bandgap, PV converter by fabricating it in
such a way as to enable removal of the InP substrate 26. For example, as
shown in FIG. 16, a monolithic, multi-bandgap (tandem), PV converter 100
is fabricated much the same as the PV converter 10 in FIG. 2 on an InP
substrate 26, except that a stop-etch layer 98 is added between the
buffer layer 32 and the front cladding layer 40 of the first subcell 22.
The stop-etch layer 98 can be, for example, n-Ga0.47In0.53As
with the same lattice constant as the InP substrate 26, so that the
subsequent layers of the first and second subcells 22, 24, tunnel
junction 44, 46, (or isolation layer for independently connected
subcells) and lattice constant transition layer 20 can be grown
epitaxially, as described above.

[0117] The purpose of the stop-etch layer 98 is to enable the InP
substrate 26 and buffer layer 32 to be removed by etching or other
selective chemical removal to create an ultra-thin, monolithic,
multi-bandgap (tandem) PV converter 100 without etching or damaging any
of the first subcell 22. After the several layers of the structure in
FIG. 16 are grown epitaxially on the InP substrate 26, the structure 100
is top-mounted on another object 102, such as a solar panel, heat sink,
printed circuit board, or other useful platform. Then the substrate 26
and buffer layer 32 are removed by etching or other selective chemical
removal, leaving the ultra-thin, monolithic, multi-bandgap, PV converter
100 mounted on the object 102, as shown in FIG. 17. The stop-etch layer
98 can be an electrically conductive material, so it can also serve as a
contact layer. If desired, part of such a conductive stop-etch layer 98
can be removed by etching or other selective chemical removal with a
different chemical in which it is soluble to leave a grid pattern, which
would be useful if the material of layer 98 is not transparent to the
incident radiation R.

[0118] Mounting the PV converter 100 on the object 102 can be accomplished
with a suitable adhesive or by any other suitable mounting mechanism. An
anti-reflective coating 97 can be added to reduce reflection of incident
radiation, or layer 97 can be any other optical control material for
purposes described above for the PV converter 10.

[0119] This ultra-thin, monolithic, multi-bandgap, PV converter 100
enables this device to be used as a solar cell, because elimination of
the InP substrate 26 allows all of the incident solar radiation SR to
reach the subcells 22, 24, which can convert it to electricity.
Otherwise, the InP substrate 26, which has a bandgap of 1.35 eV, would
absorb large amounts of solar radiation SR in wavelengths shorter
than--0.93 μm, before such solar radiation SR could reach the
first subcell 22. There is no n/p junction in the substrate 22, and it
cannot convert radiant energy to electricity, so any solar energy
absorbed by the substrate 26 would be thermalized and wasted as heat.

[0120] Even without the InP substrate, however, there could be significant
production of heat in the PV converter 100, when it is used as a solar
cell, because there is a substantial amount of energy in higher
frequencies (shorter wavelengths) of the solar spectrum, where
wavelengths are substantially shorter than the longest wavelength that
can be absorbed by the first subcell 22. Therefore, there is significant
thermalization of excess energy that is not needed for carriers to
transcend the bandgap Eg1 of the first subcell 22, thus a
significant production of heat that should be dissipated from the PV
converter 100. However, the PV converter 100 is ultra-thin and has no
thick substrate, so heat can flow through the PV converter 100 is
substantially one-dimensional, and it can flow quickly and easily to the
back surface 104. If the object 102 on which the PV converter 100 is
mounted is a good heat sink, i.e., good thermal conductivity and
sufficient mass and/or surface area to conduct heat away from the PV
converter 100, the combination provides very good thermal management and
minimizes heat build-up in the PV converter 100.

[0121] The ultra-thin, monolithic, multi-bandgap, PV converter 100 can
also be grown in a polycrystalline form on less expensive substrates,
such as graphite, which is amorphous and does not impose a lattice
constant on the first subcell 22, or in single-crystal form on compliant
substrate or bonded substrate systems, which provide a lattice constant
match to accommodate epitaxial growth. A typical compliant substrate may
be made, for example, with an inexpensive substrate material, such as
silicon, and with an amorphous oxide of the substrate material followed
by a layer of perovskite oxide. Therefore, a first subcell 22 of InP,
GaInAs, or GaInAsP will grow with its natural lattice constant. Such
first subcell 22 can then be followed by a InAsyP1-y lattice
constant transition layer 20 and another, lower bandgap, second subcell
24, as described above. Then, the resulting ultra-thin PV converter 100
is mounted on another object 102 and the compliant substrate is removed.

[0122] Compliant substrates can also be used on any of the monofacial PV
converter embodiments 10, 110, 112, 140, 150 described above, and they
can possibly be used for the bifacial embodiments 80, 160, 190, 230.
Possible uses of compliant substrates in one or more embodiments
described herein depend on the transparency and other properties of the
compliant substrate materials and systems being considered and/or
applied.

[0123] While the description of this provided in this disclosure has
focused primarily on photovoltaic converters, persons skilled in the art
know that other optoelectronic devices, such as photodetectors and
light-emitting diodes (LEDs) are very similar in structure, physics, and
materials to PV converters with some minor variations in doping and the
like. For example, photodetectors can be the same materials and
structures as the PV converters described above, but perhaps more
lightly-doped for sensitivity rather than power production. On the other
hand LED's can also be much the same structures and materials, but
perhaps more heavily-doped to shorten recombination time, thus radiative
lifetime to produce light instead of power. Therefore, embodiments also
apply to photodetectors and LEDs with structures, apparatus, compositions
of matter, articles of manufacture, and improvements as described above
for PV converters.

[0124] Since these and numerous other modifications and combinations of
the above-described method and embodiments will readily occur to those
skilled in the art, it is not desired to limit embodiments to the exact
construction and process shown and described above. For example,
accordingly, resort may be made to all suitable modifications and
equivalents that fall within the scope of the disclosure as defined by
the claims which follow. The words "comprise," "comprises," "comprising,"
"include," "including," and "includes" when used in this specification
and in the following claims are intended to specify the presence of
stated features or steps, but they do not preclude the presence or
addition of one or more other features, steps, or groups thereof. Also,
GaInAs(P) is used as a shorthand, generic term that includes any ternary
GaxIn1-xAs and/or quaternary
GaxIn1-xAsyP1-y, and similar notation conventions
apply to AlGaInAs(P).